In certain types of signalling systems, including systems on board trains, it is desirable to be able to detect a signal from a wayside station, determine its code rate, and execute an action responsive to the information indicated by the detected rate. For example, in a railway system, it is common to apply a coded a.c. signal to the track and have the signal picked up and decoded by equipment on board the train to indicate desired operating speeds. Thus, a detected signal at or near a first predetermined code rate might indicate that the train speed should be within a predetermined range, while a detected signal at or near a second code rate would indicate a different speed range. Various other types of intelligence could be carried by the coded signal. The coded signal may comprise an a.c. signal of constant frequency turned on and off at a predetermined rate for a particular code. To transmit some other code, information, or intelligence, the constant a.c. frequency would be turned on and off at a different rate. Obviously, only one coded signal could be applied to the track at any given time.
In the prior art, the coded track signals were typically detected by filters and/or tuned circuits. Such systems have been employed successfully for many years. However, the necessary filters and/or tuned circuits tend to be both bulky and expensive. Accordingly, more economical and compact equipment is disclosed herewith for detecting, decoding and responding to the coded track signal.